In 1990, the Mexican government decided to implement some air quality management programs to reduce emissions. Some of the programs included removal of lead from gasoline and the implementation of catalytic converters in automobiles, reduction of sulphur content in diesel transportation fuel, substitution of fuel oil in industry and power plants with natural gas, reformulation of liquified petroleum gas used for heating and cooking. In 1993, the government mandated the replacement of lead-octane additives with MTBE and PEMEX, the state-run petroleum company, replacing underground storage tanks at all of its petrol stations. In 2012, ozone and other air pollutants ranked at about the same level as Los Angeles. This improvement in air quality was achieved through the Mexican government's requirement that gasoline be reformulated, that polluting factories be closed or moved, and that drivers be prohibited from using their car one day per week. More recently there has been an expansion of public transportation. Air pollution has been a major issue in Mexico City for decades. Alongside a doubling of the vehicle fleet in Mexico City from 1992 to 2012, and the slow implementation of low-sulphur standards, the use of fuel ethers contributed greatly to an 86% decrease in CO, a 53% decrease in ozone, and a 32% decrease in particulate matter in that 20-year span. Sources Air pollution is defined as the presence of substances in the atmosphere that can be harmful to life when they are found in high quantities. In Mexico City's atmosphere, the pollutants are found as primary and secondary pollutants. Primary Pollutants are known as those that are emitted directly to the environment, such as Carbon Monoxide and Sulfur Dioxide. Secondary Pollutants are created in the atmosphere due to chemical reactions, such as Sulfate and Ozone. Causes and consequences of major pollutants Ozone: Ozone is formed primarily from photochemical reactions between Organic compounds and Nitrogen Oxides. Ozone has been shown to affect the respiratory, cardiovascular and central nervous system. Early death and problems in reproductive health and development are also shown to be associated with ozone exposure. Carbon monoxide: Carbon Monoxide is a gas that is mostly emitted by vehicles. Inhaling Carbon monoxide can cause illnesses like: Dizziness, brain disfunctions and even death. Sulfur dioxide: Sulfur dioxide is produces by combustion of fossil fuels .Inhaling sulfur dioxide is associated with increased respiratory symptoms and disease, difficulty in breathing, and premature death. Particulates: There are two